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August 2008

Valve fail action

By Frederick Meier and Clifford Meier

Control valves may fail in various positions-open, closed, locked, or indeterminate.

The position of a failed valve can have a significant impact on associated equipment, and therefore, it is of interest to operations personnel.

Valve fail action often is discussed and agreed upon during piping and instrument diagram (P&ID) review meetings, so it is natural and efficient to document the agreed-upon action on the P&ID.

For valve fail action, the term "Power" means the medium that moves the valve actuator and therefore the valve trim. The most common "Power" medium is instrument air. Power does not refer to the signal, unless the signal is the medium that moves the actuator.

The fail positions may be identified on the P&ID using letters below the valve symbol:

FO for Fail Open

FC for Fail Closed

FL for Fail Last or Locked

FI for Fail Indeterminate

There are other methods of indicating the fail position of control valves. Looking at the figure, an arrow up signifies the valve fails open. An arrow down is fail close. A crossing line is fail indeterminate. Two crossing lines indicate fail locked or last position.

It is important to remember that fail position refers to the loss of the primary power at the valve, the motive force. Pulling the electronic signal off the valve transducer or electro-pneumatic positioner may induce a different reaction than the failure indication shown.

A springless piston actuated valve will fail indeterminate upon loss of air. However, if there is a positioner, it will be driven in one direction or the other upon loss of the electronic signal.

An electronic loop may well control flow in a pipeline. The loop here has the number 101.

The dotted line in this loop graphic indicates information is transmitted electronically from the flow transmitter, FT-101, to the indicating controller, FIC-101, and from the controller to the current to pneumatic converter (I/P), FY 101.

FT-101 senses the differential pressure proportional to the flow rate in the line caused by FE-101, a flow element or orifice plate. FT-101 transmits a 4-20 mA dc (direct current) signal corresponding to the varying differential pressure.

FIC-101, an electronic flow indicating controller, transmits a 4-20 mA dc signal to the converter or transducer, FY-101, that converts the 4-20 mA dc signal into a pneumatic signal.

This signal changes the position of the valve actuator, which in turn changes the position of the inner works of the control valve, changing the flow through the control valve.

Notice this valve fails open, or FO.

ABOUT THE AUTHORS

Frederick Meier is a member of ISA. He worked for 50 years as a control systems engineer, teacher, and author. Son, Cliff Meier is an ISA member, a registered P.E., and a consulting engineer. Their book is Instrumentation and Control Systems Documentation, ISA Press, 2004.

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